High speed electron multiplier



cnoss REFERENCE EXAMMR AU 233 EX IPSIU May 19, 1970 G. R. FEASTER HIGH SPEED ELECTRON MULTIPLIER Filed Dec. 13, 1967 (D T h INVENTOR Gene R. Feoster BY M ATTORNEY United States Patent 3,513,345 HIGH SPEED ELECTRON MULTIPLIER Gene R. Feaster, Elmira, N.Y., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 13, 1967, Ser. No. 690,297 Int. Cl. H0lj 39/14 US. Cl. 313-95 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention This invention relates generally to electron discharge devices and more particularly to high speed photomultiplier devices.

Description of the prior art Typically, electron multipliers of the prior art took the form of an envelope in which there is disposed a photocathode element for converting incident radiation into photoelectrons, a plurality of dynode elements for successively multiplying the electrons, a coaxial termination including an anode element for collecting the electrons, and a suppressor grid disposed between the last dynode element and the anode element. It is generally desirable for the electron multiplier structure in such a device to amplify a pulse signal without producing a spread in the signal pulse width. Electron multipliers of the transmission dynode type are available and can theoretically obtain a pulse rise time of approximately 76 picoseconds if perfectly terminated. Pulse rise time may be defined as the time required for an output signal to pass from 10% to 90% of full amplitude for one electron leaving the photocathode element.

The performance of the currently available photomultipliers of the type described is limited by imperfections of the coaxial termination used to obtain the output signal of the device. One contribution to imperfect coupling arises from the finite width of the beam of radiation and the resulting electron beam which creates a dispersion effect and a resulting cancellation of a considerable portion of the high frequency output power. More specifically, the leading plane edge of the electron cloud arriving in the spacing between the anode element and suppressor grid launches an electromagnetic wave simultaneously at the center and the periphery of this space. Consequently, a phase difference occurs in the wave excited in this space, which causes a serious power loss at high frequencies.

The prior art has suggested that pulses of radiation may be directed on a photoemissive surface at a prescribed, acute angle so that the induced electromagnetic energy will be reinforced as it is propagated through a coaxial conductor. However, this solution is not readily adaptable to devices in which transmissive dynodes are employed. Typically, transmissive dynodes are of a planar configuration and are disposed in a serial relationship to successively multiply a cloud of electrons. If the termination structure of the prior art was used in conjunction with a plurality of planar dynodes, the multiplied cloud of electrons could not be used effectively to induce the electromagnetic output signal.

It is therefore an object of this invention to provide an electron multiplier device in which the degradation of the high frequency response at the output termination is significantly reduced.

It is a further object of this invention to provide an electron multiplier device which is capable of efiiciently extracting an output signal from an electron cloud that has been successively multiplied by a plurality of dynodes.

SUMMARY OF THE INVENTION Briefly, the present invention accomplishes the above cited objects by providing an improved electron discharge device including means for converting an impulse of radiation into a cloud of electrons whose leading surface has central portions which precede the peripheral portions, and means such as an interaction space disposed to receive the cloud of electrons and for converting the electrons into an electromagnetic signal. In one illustrative embodiment, the means for converting the impulse radiation include a suitable lens structure. wherein the peripheral portions are of greater thickness than the central portion in order to delay the peripheral portion of the radiation impulse, and a photocathode element for converting the delayed radiation impulse into a correspondingly shaped sheet of electrons.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention will become more apparent in view of the following detailed description and drawings, in which:

FIG. 1 shows a diagrammatical view of an electron multiplier device in accordance with the teachings of this invention;

FIG. 2 shows a sectional view of a dynode element incorporated within the device of FIG. 1;

FIG. 3 shows a sectioned view of an alternative means for providing a sheet of electrons whose leading surface has its central portions precede its peripheral portions, which may be incorporated into the device of FIG. 1; and

FIG. 4 shows an enlarged view of the suppressor grid and the anode element structure of the device of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawings and in particular to FIG. 1, there is shown an electron discharge device 10 of the photomultiplier type including an evacuated envelope 12 having a cylindrical section 30 enclosed upon one end by a faceplate 14. A photocathode element 18 is disposed upon the interior surface of the faceplate 14. Illustratively, the photocathode element 18 includes an annularly shaped electrode 20 made of an electrically conductive material which is deposited directly upon the faceplate 14 and a layer 22 of a suitable photoemissive material such as cesium antimony which is deposited thereon. A terminal 24 may be connected with the photocathode element 18 and extends through the envelope 12 as shown in FIG. 1. A pair of electrodes 26 are disposed within the cylindrical portion 30 in order to accelerate and direct the photoelectrons emitted by the photocathode element 18 onto a plurality of dynode elements 16. As shown in FIG. 1, a plurality of dynode elements 16 are disposed in a substantially parallel relationship with each l l i other in order to successively multiply the photoelectrons emitted by the photocathode element 18.

Each of the dynode elements 16 is supported within the envelope 12 by a ring-shaped flange 36 (see FIG. 2), which in turn is supported upon a ring 28. Typically, the rings 28 are made of electrically conductive material such as Kovar (a trademark of the Westinghouse Electric Corporation for an alloy of nickel, iron and cobalt), and serve as electrical terminals for applying successively greater potentials to the dynode elements 16. The annularly shaped support rings 28 are spaced and insulated from each other by a plurality of cylindrically shaped spacers 34 which are made of a suitable insulating material such as alumina ceramic. The insulating spacers 34 are disposed between the support rings 28 and are secured thereto by brazing. As shown in FIG. 1, the spacers 34 are of approximately the same diameter and are secured to the support rings 28 in order to form a portion of the envelope 12. The dynode structure is connected to the cylindrical portion 30 by flange 32 which is made of a suitable material such as Kovar. More specifically, the flange 32 is sealed to the cyindrical portion 30, and to the ring support 28 as by heliarc welding.

The electrons are accelerated by increasing potentials applied to the support rings 28 associated with the plurality of dynode elements 16. More specifically, the photoelectrons are accelerated to the first dynode element 16 where transmission secondary electrons are emitted in numbers larger than the number of incident photoelectrons. The resulting secondary electrons are accelerated to the second dynode 16 where further multiplication takes place. This process is repeated until the multiplied electrons reach the last dynode element 16. An anode element 42 is disposed to receive the multiplied sheets of electrons. A suppressor-grid electrode 38 is disposed between the last dynode element 16 and the anode element 42 to accelerate the electrons to a high velocity and to electrostatically shield the anode element 42 so that the arriving electrons will not induce a charge in the anode circuit prior to the actual entry of the cloud of electrons into the space between the suppressor'grid electrode 38 and the anode element 42. An electromagnetic wave is formed in the space between the suppressor-grid 38 and the anode element 42.

The suppressor-grid 38 illustratively takes the form of a mesh made of a suitable electrically conductive material such as nickel and having a mesh size of approximately 20 per inch. The suppressor-grid electrode 38 is mounted upon a ring-shaped flange 37 which in turn is supported upon a support plate 28. The anode element 42 forms a part of a coaxial output terminal 45. The anode element 42 is the inner coaxial member of the terminal 45 and a cylindrically shaped flange 40 forms the outer part of the coaxial terminal 45. The flange 40 is connected as by heliarc welding to the support 28 upon which the suppressor grid 38 is mounted. An interaction region 49 is formed between the suppressor-grid 38 and the anode element 42 for receiving the cloud of electrons which has been suc cessively multiplied by the transmission dynodes 16. A coaxial transmission line 46 is connected to the coaxial terminal 45 to extract the electromagnetic waves induced. in the interaction region 49. The coaxial transmission line 46 includes an outer conductor 50 which is disposed concentrically about an inner conductor 48. In order to enclose the envelope 12, an annularly shaped disc 44 is disposed between and sealed to the flange 40 and the anode element 42. Further, the disc 44 is hermetically sealed to the inner and outer conductors 48 and 50. Illustratively, the disc 44 may be made of a suitable insulating material such as ceramic alumina. A pair of electrically conductive layers 47 and 51 are applied to inner and outer peripheral portions of the disc 44 as shown in FIG. 1 in order to electrically connect the members of the coaxial terminal 45 and the coaxial transmission line 46. The layers 47 and 51 further serve to prevent the attenuation of the electromagnetic energy propagated through the disc 44 by confining the transmitted energy to that portion of the disc 44 disposed between the inner and outer conductors 48 and 50. The electromagnetic wave induced between the suppressor grid 38 and the anode element 42 is transmitted along the coaxial terminal 45 and the transmission line 46 through the disc 44 without significant change of impedance to the transmission line 46. In this way, electrical reflections that lead to poor rise characteristics are avoided and full advantage can .be taken of the rapid pulse rise time characteristics of this structure.

Referring now to FIG. 2, there is shown an enlarged cross sectional view of the dynode element 16. The dynode element 16 includes a support layer 62 of about angstroms thickness and made of a stable material such as silicon oxide. The layer 62 is disposed between two metallic rings 56 and 58, which may be made of nickel. A third ring 60 provides a means of making electrical contact to the annular flange 36 which supports the dynode element 16 upon the rings 28 as shown in FIG. 1. Next, a very thin layer 64 of a suitable electrically conductive material such as aluminum may be deposited to a weight of .01 mg. per cm. upon the layer 62. The electrons strike a layer 66 made of a suitable insulating material such as potassium chloride which is deposited upon the layer 64. The structure and the method of forming the dynode element 16 is similar to that described in US. Pat. No. 2,898,499, issued Aug. 4, 1959, to E. J. Sternglass and W. A. Feibelman The dynode element described in this patent utilizes a coherent oxide such as aluminum or magnesium oxide of the support layer, which supports a conductive layer and a secondary electron emissive layer In addition to transmission dynodes having a continuous surface, the open mesh type dynode, well known as the Weiss multiplier, is also available tor use in the present invention. This latter type dynode does not, however, provide the short rise times obtainable with transmissive dynodes and is therefore not as desirable for use in some applications.

It is an important aspect of this invention that the leading edge (or surface) of a radiation pulse 68 be converted into a sheet 72 of electrons which is of an approximately conical configuration as shown in FIG. 1. More specifically, there is provided a radiation delay means 98 which acts to delay the peripheral portions of the incident radiation sheet 68 with respect to the central portions thereof. Illustratively, the radiation delay means 98 may take the form of a lens having conically shaped surfaces 99 and 100 disposed on either side-as shown in FIG. 1. Alternatively, the lens may have one surface shaped conically, whereas the other surface is substantially planar. It is noted that the delay means may be incorporated as the faceplate 14 without departing from the teachings of this invention. The resulting delay means is thin at the central portions and its thickness increases in proportion to the radius from the center. The substantially planar leading edge of the radiation pulse 68 is directed through the delay means 98 with the result that the peripheral portions will be delayed by the thicker part of the delay means 98 presented to the radiation pulse 68. The pulse radiation transmitted from the delay means 98 has a leading surface 70 wherein the central portions precede the peripheral portions. The leading surface 70 of the radiation takes the approximate configuration of a cone and will strike the photocathode element 18 first in the center and then in an annulus of increasing radius as a function of time. In response to the incident radiation sheet 70, the substantially planar photocathode element 18 emits the cloud of electrons having the leading surface 72 of an approximately conical configuration. The leading surface 72 of electrons will pass down the length of the device 10 being successively multiplied by the plurality of transmissive dynode elements 16.

The impulse of radiation has been described in terms of its leading edge or surface. Similarly, the cloud of electrons emitted from the photocathode element 18 has been described in terms of its leading surface. It is difficult to describe with preciseness or accuracy the configuration of a pulse of radiation or a cloud of electrons. An impulse of radiation takes its form within a time period of more than seconds. During this period the leading surface may not be planar. Further, the leading surface of the radiation impulse may be planar to a degree within a tolerance of a wavelength of the radiation. Similarly, the shape of the cloud of electrons is difficult to describe due to the irregularities in the leading surface of the radiation impulse and the means used to shape the leading surface of the electron cloud. Therefore, the terms used to describe the configuration of the leading surfaces are precise to the degree with which an impulse of radiation can be observed by a resolving instrument, i.e., in the order of less than 10- seconds.

Referring now to FIG. 4, a leading surface 90 of the multiplied cloud of electrons will arrive at the suppressorgrid electrode 38 and will be inserted into the interaction region 49 between the suppressor-grid electrode 38 and the anode element 42. The leading surface 90 of the electron cloud intercepts the anode element 42 first in the center and then in an annulus of increasing diameter as a function of time. The intersection of this conical surface of electrons with the suppressor-grid 38 is a circle or more particularly an annulus of increasing diameter with time. The leading surface 90 of electrons will excite an electro-magnetic wave 94 in the gap between the suppressor-grid 38 and the anode element 42 which will travel away from the central axis of the anode element 42 with approximately the speed of light. In order that the electron cloud be inserted in synchronism with the resulting electromagnetic wave 94 the intersecting annulus of the conical surface 90 of electrons should grow in radius with approximately the speed of light. This condition is satisfied if the resulting surface 90 of electrons is of an approximate conical configuration with the central portions thereof preceding the peripheral portions. As shown in FIG. 4, the leading surface 90 is timed to enter the interaction space in such a way as to reinforce the electromagnetic wave 94 established between the suppressor-grid electrode 38 and the surface of the anode element 42. The surface 90 of electrons-reinforces the movement of the electromagnetic energy in a radial direction from the axis of the anode element 42 and propagates the electromagnetic wave 94 through a junction 96 and into the annular space between the conductor and the anode element 42.

An approximate calculation for electrons which have a speed corresponding to an accelerating field of 3600 volts shows that a cloud of electrons striking the surface of the anode element 42 with a cone apex angle of approximately 168 would require a delay means 98 having a .050 inch greater thickness at the peripheral portion than in the center if the refractive index of the glass of which the means 98 is made is 1.6.

At the present, it is believed that the discontinuity or junction 96 may present an impedance to the electromagnetic wave 94 traveling radially from the axis of the anode element 42. As a result, the discontinuity 96 may cause reflections of the radially traveling electromagnetic wave which would be reflected toward the center of the anode element 42 and which would tend to spread out again to degrade the response of this device. Thus, it may be necessary to place at the center of the anode element 42 a termination impedance 92. More specifically, the termination impedance 92 may be disposed between the anode element 42 and the suppressor-grid 38 in order to absorb the reflected electromagnetic waves. Illustratively, the termination resistance 92 may take the form of a uniform mixture of aluminum oxide and carbon 6 to provide an impedance of approximately 50 ohms between the suppressor-grid 38 and the anode element 42.

Referring now to FIG. 3, there is shown another i1- lustrative means for converting a substantially planar pulse 85 of radiation into an electron cloud having a surface 86 with a substantially conical configuration. More specifically, the photocathode element of FIG. 1 may be replaced by a photocathode element 78 having a substantially conical configuration as shown in FIG. 3. The device of FIG. 3 includes a cylindrical portion 74 having a faceplate 76 of substantially conical con figuration. The photocathode element 78 is disposed upon the faceplate 76 and includes an annular electrode 80 and a layer 82 of a suitable photoemissive material disposed thereon. The incident pulse 85 of radiation causes the photoemissive layer 82 to emit a cloud of electrons which in turn is accelerated by an electrode 84. In operation, the peripheral portions of the pulse 85 of radiation strike the photocathode element 78 to thereby emit electrons at a first point in time. At a second point in time, the central portions of the pulse 85 of radiation will strike the photocathode element 78 to emit electrons. Since, the photons of the impulse 85 of radiation travel at a faster velocity than the photoelectrons emitted from the photocathode element 78, the central portion of the cloud of photoelectrons will be farther advanced in space and will precede the peripheral portions of the surface 86.

Although it has been a common practice to provide focusing lens for such imaging devices as television camera tubes and image intensifying devices, the use of an optical delaying means such as the lens structure 98 substantially differs from such lenses. It is the object of the focusing lens to direct the radiation so that it falls simultaneously upon the photocathode element of the imaging device. In contradistinction, it is the principal object of this invention to provide means for delaying the peripheral portions of the radiation image with respect to the central portions thereof so as to produce a conically shaped sheet of electrons. As explained above, the conically shaped sheet of electrons induces and reinforces an electromagnetic wave in the coaxial terminal structure at approximaetly the speed of light to thereby improve the rise time response of this device.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A radiation sensitive device including first means for converting a substantially planar pulse of radiation into a cloud of electrons having a leading surface whose central portions precede the peripheral portions thereof, and second means for rapidly providing electromagnetic energy in response to said cloud of electrons.

2. A radiation sensitive device as claimed in claim 1, wherein said second means includes a coaxial terminal into which said electromagnetic energy is induced.

3. A radiation sensitive device as claimed in claim 1, wherein said second means includes an anode element presenting a surface transverse to said leading surface and an electrically conductive member disposed about said anode element and forming a coaxial terminal.

4. A radiation sensitive device as claimed in claim 1, wherein third means for multiplying said cloud of electrons is disposed between said first and second means.

5. A radiation sensitive device as claimed in claim 4, wherein said third means includes a plurality of transmissive dynodes.

6. A radiation sensitive device as claimed in claim 1, wherein said first means includes a substantially planar photocathode element, andsaid first ineans disposed to 7 intercept said pulse of radiation for imparting a delay to the peripheral portion of said pulse of radiation with respect to the central portion thereof.

7. A radiation sensitive device as claimed in claim 6, wherein said first means includes a lens structure whose peripheral portions are of greater thickness than the central portions thereof.

8. A radiation sensitive device as claimed in claim 7, wherein said lens structure has first and second surfaces disposed transverse to said pulse of radiation and being of substantially conical configuration.

9. A radiation sensitive device as claimed in claim 1, wherein said first means takes the form of a photocathode element whose central portions are disposed closer to said second means than the peripheral portions thereof.

10. A radiation sensitive device as claimed in claim 9,

wherein said photocathode element has an approximately conical configuration.

References Cited UNITED STATES PATENTS RAYMOND F, HOSSEELD, Primary Examiner US. Cl. X.R= 

